This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 2001-071029, filed Mar. 13, 2001; and No. 2001-072159, filed Mar. 14, 2001, the entire contents of both of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an image forming apparatus, and in particular, to an image forming apparatus in which, when printing is carried out by a plurality of print heads, a sensor reads at least two test patterns by sub-scanning one time, and a control section carries out alignment of the respective print heads on the basis of the results.
2. Description of the Related Art
Conventionally, in an ink jet printer which is a type which prints by ejecting ink from a print head having a plurality of nozzles, a so-called xe2x80x9cserial systemxe2x80x9d, which forms an image by making the print head move reciprocally along a main scanning direction (a direction perpendicular to the conveying direction of a print medium) while the print medium is conveyed intermittently (step feeding), is the mainstream system. In this xe2x80x9cserial systemxe2x80x9d, the width (the printing width of one scan of the print head) of a line of nozzles provided along the sub-scanning direction of the print head (a direction parallel to the conveying direction of the print medium) greatly affects the printing time. Accordingly, the larger the number of nozzles a print head has, i.e., the longer the head is, the shorter the printing time becomes.
However, manufacturing the long head has many problems from the standpoints of the precision of arranging the nozzles, the ink ejecting performance, high costs, and the like.
With respect to this point, by connecting a plurality of so-called short heads, which have a relatively small number of nozzles, in the sub-scanning direction, the short heads can be considered as a long head. High speed printing can be carried out in the same way as the case of using one long head.
The above describes a case of monochrome, and in the case of color, when a serial system is adopted, in order to avoid color mixing of the inks, in-line arrangement of various types (the types of ink are different) of print heads is not carried out, and the print heads are arranged so as to be offset in the sub-scanning direction (hereinafter, this will be called prior art 1).
Here, in Jpn. Pat. Appln. KOKAI Publication No. 05-278306, there is disclosed a technique of mounting a optical sensor to a carriage on which a plurality of printing elements are mounted, and detecting, by this optical sensor, a horizontal position of a vertical segment printed parallel to a medium scanning axis, and a vertical position of a horizontal segment printed parallel to a carriage scanning axis, and carrying out position adjustment of the cartridge on the basis thereof. (Hereinafter, this will be called prior art 2.).
However, in the above-described conventional technique 1, in both cases of monochrome and color, the intervals between the respective print heads disposed along the sub-scanning direction must be disposed such that the nozzle pitches of the print heads are equal intervals between the print heads as well. Namely, in the case of monochrome, if the nozzle pitch at the joint between print heads is large, so-called xe2x80x9cwhite streaksxe2x80x9d become conspicuous, and if small, so-called xe2x80x9cblack streaksxe2x80x9d become conspicuous.
Further, in the case of color, i.e., when one pixel is formed by dots of plural colors, it is expressed as a change in color tint and the color reproduceability and uniformity deteriorate.
In this way, if the positions, in the sub-scanning direction, between the respective print heads are not disposed accurately, it leads to a deterioration in the quality of the printed image, which is problematic.
On the other hand, in above-described conventional technique 2, because one test pattern is read by one scan operation, plural scan operations are needed in order to read plural test patterns. Accordingly, because advancing/retracting of the print medium conveying is repeated, the effects of errors in conveying the print medium become large, and errors in detecting the print head position as well occur easily.
Accordingly, an object of the present invention is to provide an image forming apparatus in which positional offset, in the sub-scanning direction, between plural print heads can be easily detected, and the positional offset can be accurately adjusted while reducing the effects of errors in print medium conveying.
In order to achieve the above-described object, according to a first aspect of the present invention, there is provided an image forming apparatus comprising: a plurality of heads; a scanner reading an image printed by the plurality of print heads; a main scanning mechanism which causes the plurality of print heads and the scanner to perform main scanning relative to a printing medium; a sub-scanning mechanism which causes the plurality of print heads and the scanner to perform sub-scanning relative to the printing medium; and a control section calculating an amount of positional offset of each print head in accordance with a position of the image read by the scanner, the image forming apparatus carrying out image formation by relatively main scanning and sub-scanning the plurality of print heads with respect to the printing medium, wherein
(a) the plurality of print heads form a plurality of test pattern images on the printing medium in one main scanning by the main scanning mechanism;
(b) the scanner reads the plurality of test pattern images by sub-scanning by the sub-scanning mechanism, without moving in the main scanning direction; and
(c) on the basis of results of detection from the scanner, the control section computes an amount of positional offset, in a sub-scanning direction, at the two print heads which form the test pattern images.
Further, according to a second aspect of the present invention, there is provided an image forming apparatus according to the first aspect, wherein the plurality of print heads and the scanner are fixed on a carriage which is movable in the main scanning direction by the main scanning mechanism.
Further, according to a third aspect the present invention, there is provided an image forming apparatus according to the first aspect, wherein the plurality of test patterns are printed by two print heads adjacently fixed on a carriage.
Further, according to a fourth aspect of the present invention, there is provided an image forming apparatus according to the second aspect, wherein the plurality of test patterns have horizontal lines at end portions at sides near each other.
Further, according to a fifth aspect of the present invention, there is provided an image forming apparatus according to the fourth aspect, wherein the plurality of test patterns have the plurality of horizontal lines.
Further, according to a sixth aspect of the present invention, there is provided an image forming apparatus according to the fourth aspect, wherein the plurality of test patterns each have the same number of a plurality of horizontal lines.
Further, according to a seventh aspect of the present invention, there is provided an image forming apparatus according to the sixth aspect, wherein intervals, in the sub-scanning direction, between the plurality of horizontal lines of each test pattern are constant.
Further, according to an eighth aspect of the present invention, there is provided an image forming apparatus according to the sixth aspect, wherein the plurality of test patterns are printed so as to not overlap each other.
Further, according to a ninth aspect of the present invention, there is provided an image forming apparatus according to the eighth aspect, wherein the plurality of test patterns are printed at different positions in the main scanning direction.
Further, according to a tenth aspect of the present invention, there is provided an image forming apparatus according to the ninth aspect, wherein the plurality of test patterns are printed such that the horizontal lines are contained within a range of a reading width of the scanner.
Further, according to an eleventh aspect of the present invention, there is provided an image forming apparatus according to the eighth aspect, wherein the plurality of test patterns are printed at the same position in the main scanning direction.
Further, according to a twelfth aspect of the present invention, there is provided an image forming apparatus according to the second aspect, wherein the sub-scanning mechanism has a conveying roller conveying the printing medium in the sub-scanning direction; a conveying motor applying driving force to the conveying roller; and a conveying motor control section applying a driving pulse to the conveying motor; and the control section calculates positional offset, in the sub-scanning direction, of two print heads on the basis of the results of detection of the scanner and a number of pulses of the conveying motor control section.
Further, according to a thirteenth aspect of the present invention, there is provided an image forming apparatus according to the twelfth aspect, wherein, while the scanner reads the plurality of test patterns, the conveying motor control section applies predetermined pulses to the conveying motor and intermittently conveys the printing medium in the sub-scanning direction.
Further, according to a fourteenth aspect of the present invention, there is provided an image forming apparatus according to the thirteenth aspect, wherein, when the scanner reads the plurality of test patterns, the conveying motor control section conveys the printing medium in the same direction as a printing medium conveying direction at a time of image printing.
Further, according to a fifteenth aspect of the present invention, there is provided an image forming apparatus according to the twelfth aspect, wherein, while the scanner reads the plurality of test patterns, the main scanning mechanism stops movement of the carriage.
Further, according to a sixteenth aspect of the present invention, there is provided an image forming apparatus according to the first aspect, wherein the scanner has a sensor to which three color filters of R, G, B are applied.
Further, according to a seventeenth aspect of the present invention, there is provided an image forming apparatus according to the sixteenth aspect, wherein, when two test pattern images having different colors are read among the plurality of test patterns, the scanner uses a sensor to which a color filter, which is such that a sensitivity of the test pattern image having a lowest sensitivity is made to be a highest sensitivity, is applied.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.